Mask and method for crystallizing amorphous silicon

ABSTRACT

A method of crystallizing amorphous silicon includes forming an amorphous silicon layer on a substrate, placing a mask over the substrate including the amorphous silicon layer, and applying a laser beam onto the amorphous silicon layer through the mask to form a first crystallized region, the laser beam having an energy intensity high enough to completely melt the amorphous silicon layer, wherein the mask comprises a base substrate, a phase shift layer on the base substrate, having a plurality of first stripes having a first width separated by slits, and a blocking layer overlapping the phase shift layer, having a plurality of second stripes having a second width narrower than the first width, the second stripes being parallel to the first stripes.

[0001] This application claims the benefit of the Korean PatentApplication No. P2002-060705 filed on Oct. 4, 2002, which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a crystallizing method, and moreparticularly, to a mask and method for crystallizing amorphous silicon.Although the present invention is suitable for a wide scope ofapplications, it is particularly suitable for improving fabricationproductivity.

[0004] 2. Discussion of the Related Art

[0005] A liquid crystal display (LCD) device has been in the spotlightas a next generation high value display device because of its low powerconsumption and portability.

[0006] The liquid crystal display device is composed of an arraysubstrate including thin film transistors, a color filter substrate, anda liquid crystal layer interposed between an array substrate and a colorfilter substrate. The liquid crystal display device displays images byusing transmittance of light depending on the anisotropic refractiveindex of the liquid crystal layer.

[0007] An active matrix liquid crystal display (AMLCD) device, whichincludes a thin film transistor at each pixel as a switching device, hasbeen widely used due to its high resolution and fast moving images.

[0008] In general, silicon has been used as an active layer of the thinfilm transistor. Especially, since polycrystalline silicon has a highfield effect mobility and is optically stable, it has been widely usedas an active layer of a thin film transistor for a liquid crystaldisplay device having driving circuits and thin film transistors on thesame substrate or for a display device that is much exposed to light.

[0009] Polycrystalline silicon may be formed through a high temperatureprocess or a low temperature process. The high temperature process maybe accomplished under the temperatures of about 1,000 degrees Celsius,which are much higher than the transition temperature of an insulatingsubstrate, such as a glass substrate. Therefore, the high temperatureprocess requires a quartz substrate that has a high heat resistance.However, the quartz substrate may not be cost effective for a substrateof thin film transistors. In addition, a polycrystalline silicon layerformed through the high temperature process has a high surface roughnessand comprises fine grains.

[0010] Accordingly, a method of forming polycrystalline silicon, whichincludes depositing amorphous silicon that can be formed under lowtemperature conditions and crystallizing the amorphous silicon, has beenresearched and developed. The method of forming polycrystalline siliconincludes a laser annealing method and a metal induced crystallizationmethod.

[0011] Among these methods, in the laser annealing method, pulses oflaser beams are irradiated on a substrate including an amorphous siliconlayer, and melting and solidification of the irradiated amorphoussilicon layer are repeatedly accomplished in 10 to 10² nanoseconds.Thus, damage to the substrate under the silicon layer may be minimized.

[0012] A method of crystallizing amorphous silicon will be described indetail with reference to the attached drawings.

[0013]FIG. 1 is a graph showing an energy intensity of a laser beamversus a grain size of crystallized silicon in a laser annealing method.

[0014] In FIG. 1, a first region of the graph is a partial meltingregime. Only the surface of a silicon layer is melted by the energyintensity of the first region, thereby forming small grains.

[0015] A second region of the graph is a near-complete melting regime.Grains formed in the second region are larger than those in the firstregion because the grains laterally grow. However, sizes of the grainsare non-uniform.

[0016] A third region of the graph is a complete melting regime, whereinan amorphous silicon layer is entirely melted by the energy intensity ofthe third region and fine grains are formed due to homogeneousnucleation.

[0017] Thus, in the laser annealing method, in order to form uniform andlarge grains, the energy intensity of the second region may be used, andirradiation times and overlapping ratios of the laser beams may becontrolled.

[0018] Generally, grain boundaries of polycrystalline silicon interferewith currents and lowers the reliability of a thin film transistor. Inaddition, a breakdown of an insulating layer may occur because of acollision of electrons and a deterioration in the grains.

[0019] Accordingly, the formation of single crystalline silicon isimportant, and recently, a sequential lateral solidification (SLS)method has become of interest to solve the above problems. The SLSmethod takes advantage of the fact that silicon grains grow laterallyfrom the boundary between the liquid silicon and the solid phasesilicon. The SLS method can increase the size of the silicon grains bycontrolling the energy intensity of a laser beam and the irradiationrange of the laser beam. The SLS method is disclosed in Robert S.Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc.Vol. 452, 956-957, 1997. TFTs having channel areas of single crystallinesilicon can be formed by the SLS method.

[0020]FIG. 2 is a schematic view showing the SLS crystallizing methodusing a laser annealing process according to the related art. In FIG. 2,a crystallizing mask 14, which includes slits 12 spaced apart from eachother, is disposed over a silicon layer 10 of an amorphous phase. Alaser beam 16 is irradiated on portions A of the silicon layer 10through the slits 12 of the crystallizing mask 14. The laser beam 16 hasan energy intensity that can completely melt the silicon layer 10exposed to the laser beam 16. Thus, the portions A of the silicon layer10 corresponding to the slits 12 are completely melted. Then, aplurality of grains 18 grow laterally from the boundaries of the meltedportions A of the silicon layer 10, and the growth of the grains 18 stopat region B where the grains 18 meet each other. A width from oneboundary of the portion A to the region B where the growth of the grains18 stop may be referred to as length L of the grain 18.

[0021] Although not shown in FIG. 2, the crystallizing mask 14 can bemoved laterally so that the slit 12 may overlap one of the boundaries ofthe portion A. A laser beam is irradiated on the next portion of thesilicon layer 10, which overlaps the portion A, and the next portion iscrystallized. Thus, larger grains are formed by repeatedly accomplishingthe above processes.

[0022] The processes are performed until the length of the grain isabout 10 micrometers (□), and the silicon layer including the grains maybe used as an active layer of a thin film transistor, which has achannel of about 6 micrometers (□) in width.

[0023]FIG. 3 schematically shows a mask for crystallizing according tothe related art.

[0024] As shown in FIG. 3, a plurality of blocking layers 22 spacedapart from each other is formed on a base substrate 20. Spaces betweenthe blocking layers 22 are defined as slits 24. The base substrate 20may be formed of quartz, and the blocking layers 22 may be formed ofchromium (Cr), which reflects a laser beam. The blocking layers 22 mayhave a width of about 4 micrometers (□), and the slits 24 may have awidth of about 2 micrometers (□).

[0025]FIG. 4 shows overlaps between shots of a laser beam in the SLScrystallizing process using the mask of FIG. 3. A profile of the laserbeam at each shot relates to an energy intensity of the laser beam. FIG.4 illustrates the region corresponding to only two slits of the mask forsimplicity.

[0026] As shown in FIG. 4, a first laser shot is irradiated on thesubstrate 28 including an amorphous silicon layer 26 thereon, and a beampassing through the mask has two peaks corresponding to the slits.Second, third, and fourth laser shots are subsequently irradiated suchthat peaks of each laser shot overlap each other. At this time, thepeaks overlap each other such that the overlapping portions between thepeaks can have energy intensities higher than the melting point of thesilicon layer 26.

[0027] The peaks of the first laser shot to the fourth laser shotoverlap each other with a fixed width because a position of thesubstrate 20 corresponding to the slit of the mask changes by moving thesubstrate in a first direction in FIG. 4. In FIG. 4, a second direction,which is opposite to the first direction, indicates the growingdirection of the grains.

[0028] Although not shown in FIG. 4, distance C between the two peaks ofthe first laser shot is closely connected to the width of the blockinglayer 22 of the mask 20 of FIG. 3 (i.e., a space between the slits 24 ofFIG. 3). Since the peaks of each laser shot must have the distance Ctherebetween to prevent them from overlapping each other, there is alimitation in reducing the width of the blocking layer 22 under 4micrometers (□) according to the structure of the mask of the relatedart.

[0029] More particularly, in the SLS crystallizing process using themask of the related art, if the peaks of a laser shot overlap eachother, the silicon layer is melted non-uniformly because an overlappingportion between the peaks is large, and thus the grains do not growcompletely. In addition, since a nucleation region is formed in theoverlapping portion, characteristics of crystallization get worse.Therefore, the width of the blocking layer, that is, the space betweenthe slits must be more than at least 4 micrometers (□) so that the peaksof each laser shot do not overlap each other, thereby providing areliable process. However, there is a disadvantage in that theefficiency of the process is lowered due to an increase in the lasershots.

SUMMARY OF THE INVENTION

[0030] Accordingly, the present invention is directed to a mask andmethod for crystallizing amorphous silicon that substantially obviatesone or more of problems due to limitations and disadvantages of therelated art.

[0031] Another object of the present invention is to provide a mask andmethod for crystallizing amorphous silicon to form polycrystallinesilicon having large grains.

[0032] A further object of the present invention is to provide a maskand method for crystallizing amorphous silicon for a reduced number ofprocesses.

[0033] Additional features and advantages of the invention will be setforth in the description which follows and in part will be apparent fromthe description, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

[0034] To achieve these and other advantages and in accordance with thepurpose of the present invention, as embodied and broadly described, amethod of crystallizing amorphous silicon includes forming an amorphoussilicon layer on a substrate, placing a mask over the substrateincluding the amorphous silicon layer, and applying a laser beam ontothe amorphous silicon layer through the mask to form a firstcrystallized region, the laser beam having an energy intensity highenough to completely melt the amorphous silicon layer, wherein the maskcomprises a base substrate, a phase shift layer on the base substrate,having a plurality of first stripes having a first width separated byslits, and a blocking layer overlapping the phase shift layer, having aplurality of second stripes having a second width narrower than thefirst width, the second stripes being parallel to the first stripes.

[0035] In another aspect of the present invention, a mask forcrystallizing amorphous silicon includes a base substrate, a phase shiftlayer on the base substrate, having a plurality of first stripes havinga first width separated by slits, and a blocking layer overlapping thephase shift layer, having a plurality of second stripes having a secondwidth narrower than the first width, the second stripes being parallelto the first stripes.

[0036] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiments of theinvention and together with the description serve to explain theprinciple of the invention.

[0038] In the drawings:

[0039]FIG. 1 is a graph showing an energy intensity of a laser beamversus a grain size of crystallized silicon in a laser annealing method;

[0040]FIG. 2 is a schematic view showing the sequential lateralsolidification (SLS) crystallizing method using a laser annealingprocess according to the related art;

[0041]FIG. 3 is a schematic cross-sectional view showing a mask forcrystallization according to the related art;

[0042]FIG. 4 is a schematic view showing overlaps between shots of alaser beam in the SLS crystallizing method using the mask of FIG. 3;

[0043]FIG. 5 is a schematic view illustrating the principle of phaseshift in a mask according to the present invention;

[0044]FIG. 6 is a plane view of a mask for crystallizing amorphoussilicon according to the present invention;

[0045]FIG. 7 is a cross-sectional view taken along line VII-VII of FIG.6;

[0046]FIG. 8 is a schematic view showing the energy intensity profilesof a laser beam irradiated through the mask of the present invention;and

[0047]FIG. 9 is a flow chart showing the SLS crystallizing method usingthe mask of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0049]FIG. 5 is a schematic view illustrating the principle of phaseshift in a mask according to the present invention.

[0050] In FIG. 5, a first part 54 a of a laser beam 54 passes through afirst portion M1 of a mask 50, where there is no layer, and a secondpart 54 b of the laser beam 54 permeates a second portion M2 of the mask50, where there exists a phase shift layer 52. Since the first part 54 aand the second part 54 b of the laser beam 54 go through differentoptical paths according to the existence of the phase shift layer 52,there is a phase difference ΔΦ between the first part 54 a and thesecond part 54 b passing through the mask 50. The phase difference ΔΦ isexpressed as the following equation:

ΔΦ=2 π·d(n−1)·λ,

[0051] wherein, λ represents a wavelength of a light source, n is arefractive index of the phase shift layer 52, and d represents athickness of the phase shift layer 52.

[0052] Thus, from the above equation, the phase of light can be shiftedby about 180 degrees, for example, by controlling the thickness d of thephase shift layer 52.

[0053]FIG. 6 is a plane view of a mask for crystallizing amorphoussilicon according to the present invention. In FIG. 6, a phase shiftlayer 112 is formed in a first direction on a base substrate 110. Thephase shift layer 112 includes a plurality of first stripes, each ofwhich has a first width W1. A blocking layer 114 is formed in the firstdirection, overlapping the phase shift layer 112. The blocking layer 114includes a plurality of second stripes, each of which has a second widthW2. Spaces between adjacent phase shift layers 112 become a plurality ofslits 116, and each slit 116 has a third width W3.

[0054] Here, the second width W2 is narrower than the first width W1,thereby exposing both sides D of each first stripe of the phase shiftlayer 112. The exposed sides D of the phase shift layer 112 causes aphase shift of a laser beam passing therethrough when the laser beam isirradiated, and thus profiles of the laser beam passing through the maskcan have a stiff slope.

[0055] The first width W1 may generally be twice as wide as the thirdwidth W3 in the related art. However, the first width W1 may be smallerthan or equal to the third width W3 in the present invention.

[0056] The mask of the present invention may be used for excimer laser.

[0057]FIG. 7 is a cross-sectional view taken along line VII-VII of FIG.6.

[0058] As shown in FIG. 7, the phase shift layer 112, which includes theplurality of first stripes, is formed on the base substrate 110 suchthat the first stripes are spaced apart from each other, and each firststripe has the first width W1. The blocking layer 114, which includesthe plurality of second stripes having the second width W2, is formed onthe phase shift layer 112, wherein the second width W2 is narrower thanthe first width W1, thereby exposing both sides D of each first stripeof the phase shift layer 112 by the blocking layer 114. The spacesbetween the first stripes of the phase shift layer 112 become the slits116. The slits 116 have the third width W3.

[0059] The phase shift layer 112 may be formed of a material that canreverse the phase of light, such as MoSi_(x) (molybdenum-silicide). Thebase substrate 110 may be formed of a high heat-resistant material, suchas quartz, and the blocking layer 114 may be formed of a material thatcan block a light passage, such as chromium (Cr).

[0060] In the mask of the present invention, the third width W3 of theslit 116 may be within the range of about 1 to 3 micrometers (□), andthe first width W1 of the phase shift layer 112 may be also within therange of about 1 to 3 micrometers (□). It may be beneficial that thethird width W3 and the first width W1 are about 2 micrometers (□).Accordingly, in the present invention, the resolution of the mask forcrystallizing can be improved due to a destructive interference of thebeam profile by using the phase shift layer without changing the opticalcompensating apparatus for controlling the laser beam. Therefore,productivity in the SLS crystallizing method can be increased due to themask having an improved resolution.

[0061] The exposed sides D of the phase shift layer 112 should havesizes enough so that the transmitted laser beam is reversed to haveenergy intensities larger than the melting point of a silicon layer.

[0062]FIG. 8 is a schematic view showing energy intensity profiles of alaser beam irradiated through the mask of the present invention.

[0063] In FIG. 8, a first laser shot is irradiated on a substrate 122including an amorphous silicon layer 120 formed thereon, and a beampassing through the mask has two peaks, which corresponds to the slitsof the mask. The peaks are spaced apart from each other withoutoverlapping each other. Next, a second laser shot is irradiated, whereina peak of the second laser shot overlaps the two peaks of the firstlaser shot.

[0064] In the SLS crystallizing method of the present invention, peaksof each laser shot do not overlap each other because profiles of thelaser beam passing through the mask have stiff slopes due to destructiveinterference by using the phase shift layer. Therefore, the number oflaser shots is decreased as compared to that in the related art. Inaddition, since the distance between the slits can be reduced and thenumber of slits can be increased, the resolution of the mask forcrystallizing can be improved.

[0065] Accordingly, although the mask may have the resolution of about 2micrometers (□), for example, the profiles of the laser beam do notoverlap each other, and thus the growth of grains can be stable andreproducible.

[0066] In the present invention, the number of laser shots is notlimited to two but decreased as opposed to the related art, therebyimproving productivity of the SLS crystallizing process.

[0067]FIG. 9 is a flow chart showing a SLS crystallizing process usingthe mask of the present invention.

[0068] In step ST1, an amorphous silicon layer is formed by depositingamorphous silicon on an insulating substrate and dehydrogenating theamorphous silicon to improve crystallizing characteristics. Here, abuffer layer may be formed between the substrate and the amorphoussilicon layer. The buffer layer may be formed of an insulating materialsuch as silicon oxide (SiO₂).

[0069] In step ST2, the SLS crystallizing process is performed by usinga laser. That is, a first shot of a laser beam is irradiated on thesubstrate including the amorphous silicon layer by using the mask havinga phase shift layer, and a portion exposed to the laser beam is melted.Grains grow from the boundaries of the melted portion toward the middleof the melted portion, and thus a first crystallized region is formed.The next shot is irradiated, so that the transmitted laser beam overlapsthe first crystallized region. Thus, a second crystallized region isformed.

[0070] In step ST3, a polycrystalline silicon layer is formed byrepeatedly performing step ST2.

[0071] In the mask of the present invention, the blocking layer that isformed of chromium reflects the laser beam, and the phase shift layerformed of molybdenum silicide reverses the phase of the laser beam,reducing the intensity of the laser beam. Therefore, peaks of a lasershot can be separated without difficulty.

[0072] Additionally, the resolution of the mask for crystallizing can beimproved, and thus productivity of the SLS crystallizing process can beincreased.

[0073] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the mask and method forcrystallizing amorphous silicon of the present invention withoutdeparting from the spirit or scope of the inventions. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of crystallizing amorphous silicon,comprising: forming an amorphous silicon layer on a substrate; placing amask over the substrate including the amorphous silicon layer; andapplying a laser beam onto the amorphous silicon layer through the maskto form a first crystallized region, the laser beam having an energyintensity high enough to completely melt the amorphous silicon layer,wherein the mask comprises, a base substrate, a phase shift layer on thebase substrate, having a plurality of first stripes having a first widthseparated by slits, and a blocking layer overlapping the phase shiftlayer, having a plurality of second stripes having a second widthnarrower than the first width, the second stripes being parallel to thefirst stripes.
 2. The method according to claim 1, wherein the basesubstrate has a high heat resistance.
 3. The method according to claim2, wherein the base substrate is quartz.
 4. The method according toclaim 1, wherein the phase shift layer is molybdenum silicide(MoSi_(x)).
 5. The method according to claim 1, wherein the blockinglayer is capable of reflecting a laser beam.
 6. The method according toclaim 5, wherein the blocking layer is chromium (Cr).
 7. The methodaccording to claim 1, wherein each of the slits has a width in a rangeof about 1 to 3 micrometers (□).
 8. The method according to claim 1,wherein the first width is in a range of about 1 to 3 micrometers (□).9. The method according to claim 1, wherein the first width issubstantially the same as a width of each slit.
 10. The method accordingto claim 1, wherein each of the second stripes of the blocking layer isdisposed over the middle of each of the first stripes of the phase shiftlayer, thereby exposing both sides of each first stripe.
 11. The methodaccording to claim 1, wherein the first crystallized region is formed bytwo laser shots, and the first width and a width of each slit are about2 micrometers (□).
 12. The method according to claim 1, wherein thephase shift layer has exposed portions to the laser beam, so that thelaser beam passes through the mask has a reversed profile at an energyintensity higher than a melting point of silicon.
 13. The methodaccording to claim 1, wherein the applying a laser beam is performed byan eximer laser.
 14. A mask for crystallizing amorphous silicon,comprising: a base substrate; a phase shift layer on the base substrate,having a plurality of first stripes having a first width separated byslits; and a blocking layer overlapping the phase shift layer, having aplurality of second stripes having a second width narrower than thefirst width, the second stripes being parallel to the first stripes. 15.The mask according to claim 14, wherein the phase shift layer is capableof reversing a phase of light.
 16. The mask according to claim 15,wherein the phase shift layer is molybdenum silicide (MoSix).
 17. Themask according to claim 14, wherein blocking layer is capable ofreflecting a laser beam.
 18. The mask according to claim 17, wherein theblocking layer is chromium (Cr).
 19. The mask according to claim 14,wherein each of the slits has a width within a range of about 1 to 3micrometers (□).
 20. The mask according to claim 14, wherein the firstwidth is in the range of about 1 to 3 micrometers (□).
 21. The maskaccording to claim 14, wherein each of the second stripes of theblocking layer is disposed over the middle of each of the first stripesof the phase shift layer, thereby exposing both sides of each firststripe.
 22. The mask according to claim 14, wherein the first width issmaller than or equal to a width of each slit.